This invention relates to a system for monitoring the relative displacement between one object and another object normally located adjacent to, or in close proximity of, the one object. In particular, a security system uses a position sensor employing energy in the frequency range of light, to monitor the open or closed position of a door or window, or the movement of an object such as an attache case or the like from a stored position, or a boat from a slip. The security system can also employ dual technologies such as magnetic sensing and optical sensing.
Conventionally, security systems monitoring various possible points of entry into a facility used different types of sensors to determine whether, for example, a door is positioned adjacent its associated frame (i.e., closed), or whether the movable portion of a window is adjacent its frame or a fixed portion of the window unit. Generally, these security systems use a magnetic sensor employing a reed switch or the like, and a magnet. The magnet is positioned on the door or movable portion of the window, and the reed switch on the door or window frame adjacent the magnet. When the door or window was properly positioned (i.e., closed), the strength of the magnetic field produced by the magnet is sufficient to keep the reed switch closed. When, however, the door or movable portion of the window is moved (displaced) from its closed position, the strength of the magnetic field at the location of the reed switch decreases to a level where the reed switch now opens. Opening of the switch results in a signal being sent to a monitoring device signifying the changed status (i.e., opening) of the door or window. Essentially, these security or monitoring systems detect the presence, strength, and polarity of a magnetic field as an indication that a static condition (door closed) is present.
Various attempts have been made to defeat these systems. One approach in doing so has been to introduce a second magnet next to the reed switch to "fool" the switch when the door or window is displaced. By doing so, the security system is tricked into indicating that the door or window is still closed; although in reality, the door or window has been opened to permit unauthorized entry. To thwart these attempts, improvements have been made to the security systems. One such improvement is the use of multiple magnets per sensor (reed switch), and multiple sensor/magnet combinations, so that merely altering the magnetic field by locating another magnet adjacent the switch is not sufficient to keep the switch from triggering a monitor when the door or window is opened. Regardless of the nuances employed in the different ways by which magnetic sensing based security systems have been upgraded, it has nonetheless been found that some magnet/switch combinations can still be compromised; while others have proven to be so unstable that they cannot be relied upon in system usage. It is therefore desirable to have a security system and system sensor which is not defeatable so that the facility's security cannot be compromised.